Most prior vehicle alarms are basically motion or acceleration sensors, vibration sensors, or both. See, for example, U.S. Pat. No. 4,817,497 to Howell, et al.; U.S. Pat. No. 4,479,114 to Yamamoto; U.S. Pat. No. 3,864,675 to Colibert; U.S. Pat. No. 4,013,995 to Adamo; U.S. Pat. No. 4,311,891 to Faust; U.S. Pat. No. 4,972,713 to Iwata; U.S. Pat. No. 3,721,756 to Hamann, et al.; and U.S. Pat. No. 3,858,131 to Larsson. Experience has shown that many such sensors are prone to false alarms when the sensor is set to detect small movements or ineffective when the sensor is set to only respond to a high threshold of motion. Most of these alarm systems employ, as the sensor mechanism, a moveable mass attached to an electrical device which produces a signal (by making an electrical contact) when the mass is moved sufficiently far from its equilibrium position. Most of these structures include a spring (or an element which functions as a spring and which holds the mass at, and which tends to restore the mass to, its equilibrium position. When the sensor mechanism is moved by a sufficiently large acceleration, a circuit is closed (or opened) and momentary electrical signal is generated (or a constant electrical signal interrupted), thus triggering an alarm.
Many of these alarm systems, for example Harmann, et al., Adamo, Howell, et al. and Yamamoto, use a latching circuit to trigger an alarm as soon as a single contact closure occurs. Such systems are especially prone to frequent false alarms as the contact closure is often the result of only a slight motion or vibration not caused by an intrusion or violation attempt.
An important consideration in the design of such prior art alarm systems is the tradeoff between sensitivity and false alarms. In the past, the designer frequently had to compromise between providing an alarm which was sufficiently sensitive to protect the vehicle (or structure) in question, while not responding to motions and vibrations, such as would be generated, for example, by innocent jostling of a vehicle, applying pressure on a bumper or fender or even high winds or heavy rains.
Portable alarms, i.e., alarms which are temporarily installed in a vehicle for security while it is parked, for example in or near a repair garage or truck depot, are known, for example, from U.S. Pat. No. 3,668,675 to Joens, et al. and U.S. Pat. No. 4,187,497 to Howell, et al. However, in practice, such "portable" alarms have proven cumbersome to use and ineffective in that, like the alarm systems discussed above, they relied either on contact closure latching circuits to sound an alarm, or vibration sensors which were too sensitive for practical use, in either case resulting in frequent false alarms and/or the need to reset the device each time it sounded an alarm. Moreover, many require that the contact arms be horizontally oriented, making them cumbersome to use for vehicles parked on non-level terrain.
Such cumbersome and false-alarm-prone devices are not suitable for effective theft and intrusion prevention by mechanic shops, service stations, fleet owners, e.g., rental companies and delivery companies, transit corporations, or the like, who frequently have a need to economically and efficiently secure and protect various different vehicles which may be parked in a high-risk situation, or which contain valuable cargo, e.g., vans, delivery trucks, repair trucks, etc. Additionally, none of the previous systems take advantage of the integral, time-limiting input circuitry required of many transmitters used in the security industry for monitoring non-latching contact sensors.
As used in this specification, the term "detection circuit" is used to refer to a circuit which recognizes a predetermined minimum number of switch contact closures within a predetermined time interval or a single switch contact closure event lasting at least a predetermined time interval.